A significant problem in CO.sub.2 lasers is the decomposition of the CO.sub.2 in response to the impact of electrons that are utilized to excite the CO.sub.2 molecules, such as by the following reactions: EQU CO.sub.2 +e.fwdarw.CO+O.sup.- ( 1) EQU CO.sub.2 +e.fwdarw.CO+O+e (2)
where "e" represents an electron in the electrical discharge through the CO.sub.2 that excites these gas molecules into excited states from which photons can be emitted as part of the lasing process. Unfortunately, these reactions can result in more than 60% of the CO.sub.2 within the laser being decomposed. This results in a loss of power and gain and, for small lasers, can even prevent lasing.
Several approaches have been adopted to address this problem. In one such class of lasers, a fresh supply of the gas mixture (typically consisting of CO.sub.2, N.sub.2, He) continuously flows through the laser chamber. However, because this mixture is approximately 80% helium, such lasers exhibit a significant rate of helium consumption (on the order of 100 liters/hour for a 1000 Watt laser), which is not only costly, but a wasteful use of a limited resource. The helium is included, because its small atomic mass and inert chemical activity make it ideal for conducting heat away from the region within which the CO.sub.2 is induced to lase (i.e., the "lasing region").
In a second class of CO.sub.2 lasers, this gas is pumped past a heated catalyst, such as platinum, located external to the laser chamber to regenerate the CO.sub.2. Unfortunately, in such lasers, about 10% of the gas must still be dumped in each cycle. In a third class of CO.sub.2 lasers, attempts have been made to include such a heated catalyst within the laser chamber. However, such heated catalysts increase the gas temperature to a level that prevents lasing. In a fourth class of CO.sub.2 lasers, the laser gas is passed through an ambient temperature granular catalyst (such as platinum on tin oxide, Hopcalite or cobalt oxide) located outside of the laser chamber.
In U.S. Pat. No. 4,756,000 entitled Discharge Driven Gold Catalyst With Application To A CO.sub.2 Laser, issued on Jul. 5, 1988 to John A. Macken, a gold catalyst is coated onto the inside surface of the laser chamber wall to catalyze reconstitution of the CO.sub.2. In order to prevent this layer of gold from shorting out the discharge process, this gold layer is divided into electrically insulated islands of gold, of length (along the axis of the cylindrical laser chamber) preferably less than half the diameter of the cylindrical laser chamber. Alternatively, the gold is deposited as "microscopically divided gold" (i.e., microscopic granules of gold) on the inside surface of the laser chamber wall or the gold is processed to form such granules after deposition on the laser chamber wall. Analogous lasers employing a silver oxide catalyst are presented in an associated patent application entitled Discharge Driven Silver Oxide Catalyst With Application To A CO.sub.2 Laser.
Lasers typically exhibit a peak efficiency (i.e., the ratio of output power to input power) as a function of input power and therefore are normally operated at such optimal input power. Fabry-Perot type fluid lasers are typically operated in a TEM00 mode, because this cylindrically symmetric mode of laser light can be well focused by conventional optical elements. The main thermal effect in a gas discharge is the transfer of thermal energy from the electrons to translational and rotational energy of the gas that produces the inversion in the laser.
For the following reasons, lasers typically exhibit a power that varies linearly with the length of the cylindrical lasing region and is substantially independent of the diameter of the lasing region. For the electron temperature profiles of two discharges to be similar, it can be shown that the power per unit length (i.e., E.sub.A .cndot.I, where E.sub.A is the axial electrical field strength and I is the axial current) must be the same for both discharges. The power of such a laser is therefore proportional to its length.
A corresponding similarity law for the diameter D of the cylindrical laser cavity to have optimum electron energy distribution requires that the product E.sub.A .cndot.D be the same for both discharges. In combination with the above relation between E.sub.A and I, this requires that the ratio I/D be the same for both discharges. Because of these relations, the power of a TEM.sub.00 mode can be as high as for a higher order mode. A typical CO.sub.2 laser has a 9 mm inside bore (i.e. 9 mm inside diameter) and a length on the order of 1.3 meters and exhibits a power of approximately 40 Watts/m times the length of the lasing region.
In certain applications in medicine, such as removing freckles, it would be advantageous to have a handheld laser that can be comfortably held by a technician and can produce a beam of approximately 5-10 Watts. Unfortunately, a conventional 10 Watt CO.sub.2 laser exhibiting a typical 20% efficiency is approximately 25 cm long. For convenience of use, such a laser should have a length on the order of the length of a person's palm (on the order of 12 cm). Therefore, it would be very useful to produce a CO.sub.2 laser exhibiting at least twice the output power per unit length of conventional CO.sub.2 lasers.